When Melkor had ceased his attacks after Tulkas's arrival. The Valar spent their time in peace and continued rebuilding the world. The Two Lamps that were erected and brought light to the world were in need of guardianship. So Manwe, the King of Arda, called for the Spirits of Arda to answer.
First and foremost to answer the call was Nan and Gilim, who both embodied Summer and Winter respectively. Gilim would gain guardianship over the northern lamp of Illuin, while Nan would take up guardianship over Ormal. During this time, Leukë spoke to the indigenous Spirits of Arda and made a pact with those willing to act.
The Guardians began utilizing and training beasts (Dinosaurs) for battle in case Melkor and his forces would return. For thousands of years they trained and domesticated many dinosaurs for battle and at last, a sizable army was placed at Illuin and Ormal. The Valar and their servants at Almaren rested peacefully knowing that their efforts bore fruit and were guarded.
Sickness came upon many Dinosaurs and Arda seemed to be stricken with plague. Many Dinosaurs became ill and drew the attention of many Guardians to their animal companions--without warning, Melkor struck out at the Two Lamps in a surprise attack--his spies had told him that Tulkas was resting and Melkor knew that this would be the opportune moment to strike.
Melkor came out in war with many of his dark servants. The Guardians fought long and hard, but in the end they were not match for Melkor and his servants powers--and the Lamps were destroyed causing a fiery maelstrom that wiped the race of Dinosaurs out and nearly all The Guardians and neutral Spirits of Arda.
As Tulkas came forth, and the forces of Almaren came out to contend with Melkor--he and his forces fled, leaving the pieces of the world behind to be repaired by the Valar. The Valar did so, but knew that the world and all that they had created had been marred. Thus, the Valar removed themselves to Valinor across the sea of the broken world while Melkor waited and plotted his next move.
Of what became of the Dinosaurs and The Guardians, some loremasters say that some still exist as part of Valinor, others say that it was Melkor's spies who retrieved the Dinosaur's eggs for dark purposes, in truth the answer lies in both, for those Guardians who fought for the Valar were reborn in the Undying Lands along with their animal companions--but the eggs that Melkor's agents stole soon gave Melkor the idea to create Fell Beasts, and other Uvanimor.